1. Field of the Invention
The present invention relates in general to a method of etching sendust, and more particularly to a technique for effectively etching a sendust film formed on a substrate. The invention is also concerned with a method of pattern-etching sendust and chromium films, and more particularly with a technique for effectively etching a chromium base film to form a pattern which precisely conforms to a predetermined sendust pattern formed by etching the sendust film.
2. Discussion of the Related Art
Sendust, i.e., Fe--Si--Al alloy, is well known as a metallic magnetic material, and has been used for various applications. For example, sendust is used for forming a metallic magnetic layer which is provided in a magnetic gap of a composite type magnetic head core, as disclosed in U.S. Pat. No. 5,084,129. This magnetic head core having a so-called metal-in-gap (MIG) structure is suitably used for recording information on a magnetic recording medium having a high coercive force.
As one of a series of process steps for fabricating the composite type magnetic head core of the above type, for example, a sendust film with a suitable thickness is formed on a suitable substrate by sputtering or other technique, and is then subjected to a chemical etching or other treatment. In this process step, it is particularly important to form the sendust film into a predetermined pattern with high accuracy, by utilizing a photolithography technique in combination with the etching treatment.
In the meantime, there have been only few reports on methods of etching a sendust film formed on a substrate. For example, a sendust material may be etched by using a solution of phosphorous acid (10%) and pure water, as proposed in JP-A-61-104307, or using an etching liquid which consists of a 30% nitric acid solution, as proposed in JP-A-3-237605.
Having studied the above etching methods, however, the inventors of the present invention found that the use of the solution of phosphorous acid (10%) and pure water involves some problems, such as a slow etching speed, low stability and poor reproducibility. It was also recognized by the inventors that the use of the 30% nitric acid solution for etching sendust involves some problems, such as a considerably slow etching speed, e.g., about 0.0053 .mu.m/min., and poor conformity of edges of etched portions of the sendust film to those of the predetermined pattern.
In order to improve the adhesion of the sendust film with respect to the substrate which bears the film thereon, it is proposed in JP-A-2-295104 to form a base film, such as a chromium film, on the substrate, and then form the sendust film on the base film. In this case, the chromium base film as well as the sendust film needs to be patterned by etching, such that a pattern obtained from the base film conforms to or coincides with a pattern obtained from the sendust film. Namely, after the sendust film formed on the base film is etched to form a predetermined pattern, the chromium base film is etched into a pattern corresponding to that of the sendust film.
However, the chromiun base film tends to be etched to a larger extent than expected whereby the pattern formed by the base film is made smaller than the predetermined sendust pattern. In some cases, the chromium base film located under the sendust pattern is excessively etched, whereby the patterned sendust film may be undesirably peeled off or separated from the base film.